JPS63308617A - Fine constant current circuit - Google Patents

Abstract

PURPOSE:To obtain an arbitrary fine constant current from the output of a current mirror circuit by connecting a photo diode to the input terminal of the current mirror circuit by making it to a reverse bias. CONSTITUTION:In order to control the current of an LED2, one edge of the LED2 is grounded through a variable resistance R16 to a terminal 7 connected to an LED driving power source V1 and other edge is grounded. A resistance value R1 is made variable and the light emitting output of the LED2 is controlled. As the result, the output current of a photo diode 3 connected to the collector of a transistor TR4 and a bias power source terminal 8 is changed. The current comes to be the collector current of a TR5 and is transferred by the current mirror circuit composed of the TR4 and 5. Consequently, by the control of the resistance value R1, an arbitrary fine constant current can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a minute constant current circuit, and particularly to a minute constant current circuit having a current mirror circuit capable of generating a minute current.

[Conventional technology]

Conventionally, when trying to obtain a constant current source with high precision, an operational amplifier (hereinafter referred to as an operational amplifier) and a transistor have been used.

FIG. 3 is a constant current circuit diagram showing an example of such a conventional device.

As shown in FIG. 3, the output terminal 19 of such a constant current circuit
The output current obtained from the output current is calculated by comparing the voltage V, 5- of the reference voltage source 16 input to the operational amplifier 15 with the voltage on the emitter side of the output transistor 17, and calculates the change in pace current by calculating the change in the collector current of the transistor 17. Obtained as a change. Its power value engineering.

It is given in free. Here, α is the ground current amplification factor of the transistor 17, s is the reference voltage, and 3 is the constant current source that is adjusted to obtain a minute constant current of the constant current source adjustment resistor 18 to obtain a predetermined constant current. The resistance value R3 of the resistor 18 is set large,
For operational amplifiers, MO8 with low input bias current
It is necessary to select an input type or a J-FET input type.

[Problem that the invention seeks to solve]

However, in such a constant current circuit, for example, 10
0 t) To obtain a constant current of A, the reference voltage VB = l
In the case of v, a resistance of is required. Here, by using a transistor whose current amplification factor α is in the low current region 1,
= 1.

Therefore, it is difficult to realize a variable resistor with a high resistance of 104 MΩ, so the constant current circuit shown in Fig. 3 has the disadvantage that it is not possible to obtain a constant current source that outputs continuously from pA. Ta.

On the other hand, from the above equation (1), the resistance value R3" of the variable resistor 18
l'Tl sweat ILIO(J+J (lJ(JtJJL
For example, fix I%≦=/ to 100MΩ, and set l and V
If s is made variable, a continuous constant current source can be obtained, but the smaller the current, the smaller the value of the reference voltage V, and the input voltage of the operational amplifier must be controlled using a potentiometer. There was a drawback.

For example, to fix R3=100MΩ and obtain a constant current source of 10[)A, the reference voltage Vs=100MΩ×101
01) Must be controlled so that A=1. Therefore, as for the operational amplifier used here, it is necessary to select an operational amplifier with low offset v) voltage fluctuation, which is a big problem in any case.

[Means for solving problems]

The micro constant current circuit of the present invention includes a photocoupler consisting of a light emitting diode connected to a drive power source via a variable resistor, a photodiode connected to a bias power source, and a photocoupler connected to the photodiode by shorting the collector base. With one transistor.

a current mirror circuit consisting of a second transistor whose collector is connected to the output terminal, the photodiode connected to the input terminal of the current mirror circuit is connected in reverse binary, and the output of the current mirror circuit It is configured so that a minute constant current can be obtained from the terminal.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a micro constant current circuit diagram showing a first embodiment of the present invention.

As shown in FIG. 1, it has a photocoupler 1 and a current mirror circuit consisting of two transistors 4 and 5, and a photodiode 3 is connected in reverse bias. In other words, the photodiode 3 utilizes its excellent input/output characteristics in which the optical input vs. output current linearity is maintained at several pA''1 during reverse bias.On the other hand, the photocoupler 1
is a light emitting diode (LED) 2 and a photodiode (pD
) 3, and is housed in a coupler structure covered with a light-shielding material to eliminate the influence of external light. Also, NPN
) The transistor 4.5 constitutes a current mirror circuit as described above, and the input terminal of this current mirror circuit, that is, N
The shorted collector-base terminal of the PN transistor 4 is connected to the anode of a photodiode (1)D)3. Also, the cathode of photodiode 3 is approximately 0.7
It is connected to a photodiode bias power supply terminal 8 to which a voltage of 7 or higher is applied. As a result, the photodiode 3 is maintained in a reverse bias state, and a minute constant current is obtained from the output terminal 9 connected to the output of the current mirror circuit, that is, the collector of the NPN transistor 5.

Next, the operation of the minute constant current source will be explained in some detail along with the operation of the photocoupler 1. First, light emitting diodes (LEDs)
)2, one end of LED2 is connected to the LED
A variable resistor (R1) is connected to the terminal 7 connected to the drive power supply (vl).
)6, and the other end is grounded. By making the resistance values R and t of the variable resistor 6 variable, 9 LEDs
Controls the light emission output of 2. As a result, the output current of the photodiode 3 connected to the collector of the transistor 4 and the bias power supply terminal 8 changes. The current is transmitted as a collector current of transistor 5 by a current mirror circuit consisting of transistors 4 and 5. In short, since the collector current of this transistor 5 becomes a minute constant current source, any constant current can be obtained by controlling the resistance value R of the variable resistor 6. This is called a so-called suction type minute constant current circuit.

Next, FIG. 2 is a micro constant current circuit diagram showing a second embodiment of the present invention. The embodiment shown in FIG. 2 is an example of a so-called sucking type minute constant current circuit.

As shown in FIG. 2, this minute constant current circuit differs from the circuit shown in FIG. - The terminal of photodiode 3 connected to the base has become a cathode. Further, it is desirable that a relatively large voltage be applied to the terminal 12 connected to the bias power supply (3) of the photodiode 3 in order to widen the voltage range of the micro constant current circuit. As a result, the bias voltage of photodiode 3 is (V3
−VBE). Note that VBE is the base-emitter voltage of the PNP transistor 10.

The operation of the minute constant current circuit described above is almost the same as that of the embodiment shown in FIG.
The only difference is that a drain current is output from the collector to the output terminal 9.

〔Effect of the invention〕

As explained above, the micro constant current circuit of the present invention uses an inexpensive photocoupler consisting of a light emitting diode and a photodiode, without using a high resistance or low bias current type operational amplifier, a potentiometer, or a high precision, low bias current type operational amplifier. and a current mirror circuit consisting of a transistor, and by connecting the photodiode with a reverse bias to the input terminal of the current mirror circuit, it is possible to obtain a minute constant current of up to several pAg from the output terminal of the current mirror circuit. effective. In addition, since the control side and output side of such a constant current circuit are optically coupled, there is an effect of being strong against electromagnetic noise and electrostatic noise. Since the circuit can be placed far away from the output side, it also has the effect of making the circuit easier to handle.

[Brief explanation of drawings]

Fig. 1 is a minute constant current circuit diagram showing a first embodiment of the present invention, Fig. 2 is a minute constant current circuit diagram showing a second embodiment of the present invention, and Fig. 3 is a constant current circuit diagram showing a conventional example. It is a current circuit diagram. 1...Photocoupler, 2...Light emitting diode (LED), 3...Photodiode (PD)
), 4. 5...NPN type transistor, 6...
Constant current source adjustment variable resistor, 7...LED drive power supply terminal (Vl), 8...PD bias power supply terminal (v2), 9...output terminal, 10. 11...
...PNPfi) transistor, 12...Bias power supply terminal (v3). Agent: Patent Attorney Susumu Uchihara 1st I! 1 Helmet 2 Illustration Ko 3 Ward

Claims (1)

[Claims] A photocoupler consisting of a light emitting diode connected to a drive power source via a variable resistor, a photodiode connected to a bias power source, a first transistor connected to the photodiode with a collector-base short circuit, and an output terminal. and a second transistor having a collector connected to the current mirror circuit, the photodiode connected to the input terminal of the current mirror circuit is connected with a reverse bias, and the photodiode is connected to the input terminal of the current mirror circuit, and A minute constant current circuit characterized by being configured to obtain a minute constant current.